1. Field of the Invention
The present invention relates to a diamond field-effect transistor which is useful for high-temperature, high-power and high-frequency electronic devices, more particularly to a highly-oriented diamond film field-effect transistor (FET) utilizing highly-oriented diamond films which have characteristics similar to single crystal diamond.
2. Prior Art
Diamond has excellent characteristics such as high thermal conductivity (20 W/cm.K), large band gap (5.5 eV) and high electron and hole mobilities (electron: 2000 cm.sup.2 /V.s, hole: 2100 cm.sup.2 /V.s), and therefore diamond is expected to be used in various fields such as electric devices which are operative under high temperature and irradiation, high-power and high-frequency devices and the like.
FIG. 1 shows a prior art field-effect transistor utilizing a diamond film. The FET shown in FIG. 1 is a metal-semiconductor junction type field-effect transistor (MESFET) (Japanese under Provisional Publication hei 3-94429) illustrating depositing a P-type semiconducting diamond film 42 on a diamond substrate 41 as a channel layer and subsequently forming each of a source electrode 43 comprising a Au/Mo/Ti multilayer, a gate electrode 44 comprising Al and a drain electrode 45 comprising a Au/Mo/Ti multilayer on the P-type semiconducting diamond film 42.
The source-drain characteristics of this MESFET are shown in FIG. 2, where the drain current is plotted versus the drain voltage. In FIG. 2, V.sub.g indicates the voltage applied to the gate electrode 44. By applying the positive bias to the gate, the source-drain current is controlled as shown in FIG. 2 (H. Shiomi, Y. Nishibayashi and N. Fujimori, Jpn. J. Appl. Phys., Vol. 29, No. 12, page L2153, 1989).
In order to reduce the leakage current from the gate electrode in the MESFET, a metal-intrinsic semiconductor FET (MISFET) has been proposed (Japanese under Provisional Publication hei 1-158774) in which an insulating diamond layer 46 is inserted between a channel layer 42 made of a semiconducting diamond film and the gate metal electrode 44 as shown in FIG. 3A. This MISFET device shows the FET response as shown in FIG. 3B (N. Fujimori and Y. Nishibayashi, Diamond and Related Materials, Vol. 1, P665 (1992)). FIG. 3B is a graph showing the source-drain characteristics of the drain current versus the drain voltage. In FIG. 3B, V.sub.g also indicates the gate bias.
Also, in Japanese under Provisional Publication hei 3-263872, in order to reduce the leakage current from the gate electrode, a field-effect transistor having a MIS structure has been proposed as shown in FIGS. 4A and 4B.
FIG. 4B is a plan view showing this device. FIG. 4A is a cross sectional view showing the enlarged region between A and B of FIG. 4B.
These electrodes are provided so as to enclose a circular drain electrode 57 with a ring-shape gate electrode 55 and to further enclose the gate electrode with a source electrode 56. The drain electrode 57 and the source electrode 56 consist of Au/Ti bilayer films and the gate electrode 55 consists of an Al film.
In this FET, an undoped insulating diamond film 52 is formed on a Si.sub.3 N.sub.4 substrate 51 and a B-doped P-type diamond film 53 is formed on the undoped diamond film 52. The gate electrode 55 is formed on the B-doped diamond film 53 through an insulating diamond film 54 and the source electrode 56 and the drain electrode 57 are formed directly on the B-doped diamond film 53.
As mentioned above, the MIS structure comprising the Al gate electrode 55, the undoped insulating diamond film 54 and the B-doped semiconducting diamond film 53 is formed in the gate region.
FIG. 5 shows the current-voltage characteristics of this field-effect transistor (Nishimura, Kato, Miyauchi and Kobashi, The Proceedings of The 5th Symposium on Diamond, p 31, 1991). In FIG. 5, the x-axis indicates the drain voltage (V) and the y-axis the drain current (.mu.A). FIG. 5 shows the field-effect characteristics of the diamond FET shown in FIGS. 4A and 4B.
In Japanese under Provisional Publication hei 3-12966, a FET having an insulating layer 64 inserted between a P-type semiconducting film 62, formed on a substrate 61, and a gate electrode 65 has been proposed as shown in FIG. 6. A numeral 63 is a drain electrode and 66 is a source electrode. The P-type semiconducting film 62 is a B-doped diamond film. The insulating layer 64 is made of silicon oxide.
FIGS. 7A and 7B show the source-drain characteristics of the FET utilizing SiO.sub.2 for this insulating layer 64 (A. J. Tessmer, K. Das and D. L. Dreifus, Diamond and Related Materials, Vol. 1, p. 89, 1992; G. G. Fountain et al., Diamond Materials, p. 523, The Electrochemical Society 1991).
Although modulation of the drain current by the gate bias can be seen in the prior art FETs utilizing diamond, no FET, which has sufficient pinch-off and saturation characteristics to be practical, has been realized yet.
One reason for this is that the leakage current from the gate electrode increases when positive bias was applied to the gate electrode. Therefore, a sufficient depth of the depletion layer is not generated in the P-type semiconducting channel diamond layer. In order to generate the depletion layer in the entire P-type channel layer, it is necessary to reduce the doping concentration or to make the channel layer very thin. However, it is very difficult to form a thin P-type channel layer with good reproductivity by chemical vapor deposition. Further, there is a defect that, if the doping concentration is reduced, the resistance between the source and drain electrodes increases and therefore it is difficult to obtain high FET characteristics.
FETs can be fabricated on a single crystal diamond substrate. However, it is very difficult to form many devices on the substrate since a single crystal diamond substrate typically has a very small surface area, FETs can also be fabricated on a diamond film deposited on non-diamond substrates such as silicon wafers. However, in this case, the diamond film is polycrystalline so that the carrier mobilities are very small due to the existence of grain boundaries. Therefore, the transistor characteristics are usually very poor.